Difference Between Genes And Mutations
The genes of a given chromosome can be pictured as grouped together in a string. Each gene occupies a definite place in the string. Genes, together with the environment, are responsible for the development of a living body from fertilization through various stages of life—infancy, youth, maturity, old age, and death.
What roles do different genes play in this process? Genes are like separate players in a symphony orchestra. A symphony is the product of all the players working together. Yet each player has a different function. When a gene changes, the result may affect various characters or traits. In a human, for example, some gene changes influence the color of the eyes. Others impact skin color or the shape of the nose. Still others dictate the speed of blood-clotting or the level of intelligence.
Curiously enough, the laws governing the inheritance of specific characteristics (traits) were worked out long before scientists knew anything about either genes or chromosomes. In 1865, Gregor Mendel produced a 20,000-word paper about his experiments in the crossbreeding of common garden peas. In his paper, he established certain laws of heredity that have come to be known as Mendel’s laws. Scientists at that time were not particularly impressed, and Mendel’s paper was ignored for decades. It was not rediscovered until 1900. Scientists later learned that Mendel’s laws regarding peas also applied to other organisms, including human beings.
Mendel’s laws are more easily understood after knowing some basic facts about genes. Ordinarily, each inherited trait is controlled by a specific pair of genes. One was contributed by the mother and one by the father. These genes are always found in the same relative position on a particular pair of matching chromosomes. But some traits (such as susceptibility to cancer) are determined by the interaction of many pairs of genes. The gene pairs may even be located on different chromosome pairs.
Geneticists sometimes speak of an organism that is pure with respect to some trait. This means that both genes for the trait are identical. Consider the genes for hair texture—curly or straight. In an individual with a pure trait, both genes would be the same—two genes for curly hair or two for straight hair. As a result, when two pure organisms mate, all their offspring will show this trait.
A hybrid is an organism that possesses two different genes for a particular trait. If two hybrids are crossed, some of their offspring will show the effects of one gene, and some will show the effects of the other. An individual with a hybrid trait for hair texture would have one gene for curly hair and one gene for straight hair. In this case the person’s hair would actually be curly. This is because the gene for curly hair is more powerful in determining this trait than is the gene for straight hair. The gene for curly hair is said to be dominant and the gene for straight hair is said to be recessive.
Cells and chromosomes are continually dividing in the body, and mistakes occur frequently. Cells have a variety of mechanisms for correcting such mistakes, and most are repaired. Occasionally, however, one of the genes of a sex cell will no longer be an exact copy of the original gene. A change of this sort is called a mutation, and the changed gene is known as a mutant. The rate of such mutations can be increased by radiation, chemicals, and viruses, among other things.
A small minority of mutant genes may produce desirable effects and will be passed on to the organism’s progeny. That is one essential element in the process by which evolution occurs. Most mutant characters, however, are harmful, lessening the chances of survival. Many of these are immediately lethal and lead to miscarriages. Others cause serious illnesses that greatly shorten life expectancy. Spinal-cord defects, such as spina bifida, are one such disorder. Another mutation results in limb deformations.
Some mutant characters do not produce illness until later in life, allowing the carrier of the defective gene to marry and pass on the defective gene to progeny before the disorder develops. Such disorders include Huntington’s chorea, multiple sclerosis, and amyotrophic lateral sclerosis, also known as Lou Gehrig’s disease. Perhaps as many as 4,000 diseases are caused by genetic defects.
Surprisingly, some mutations can be both helpful and harmful, as is the case with sickle-cell disease, which strikes primarily blacks and can lead to life-threatening sickle-cell crises during periods of stress. The gene that causes sickle-cell disease is beneficial to blacks living in Africa because it confers resistance to malaria. But in areas where malaria is not a threat, this gene is a liability.